Clean technology is the application of the environmental sciences and engineering knowledge to conserve natural environment and resources. This emerging sector deals with products and processes that harness renewable energy sources, reduce the depletion of natural resources including water supply, and minimize pollution and waste.
With the ever increasing shortage of energy and water, clean technology may offer many opportunities for innovations, business markets and job creation. For example, water is often used for washing or cleaning of automobiles, house windows and high-rise building windows. Self-cleaning coatings on glasses which may no longer need the use of washing water can be one of the approaches to reducing water consumption.
The science and technology of self-cleaning coatings have been advancing in recent years. Their potential as a commercial product can be huge yet their global market has not been fully exploited. Potential applications of self-cleaning coatings may range from window glass, building exterior, textiles for reducing the usage of water, cleaning chemicals, and labor costs, to processes and materials for enhancing safety/reliability in transportations and communications by minimizing dirt or snow accumulations on airplane cockpit windows, wings and telecommunication antennas.
There are two different technical approaches for self-cleaning coatings—hydrophobic vs. hydrophilic. These two types of coatings both clean themselves through the action of water. The underlying mechanism in the case of hydrophobic surface is the rolling droplets taking away the dirt and dust while the sheeting water carrying away dirt is the mechanism for hydrophilic surface. Nature prefers hydrophobic self-cleaning coatings like Lotus leaf to hydrophilic coatings. There are more than 200 species of plants known to utilize superhydrophobic rolling droplets of water to keep clean.
Hydrophilic coatings, if a TiO2 type material is used, have an additional property of photocatalytic breakdown of adsorbed organic dirt by UV (sunlight) illumination, which is then washed away when it rains. For environments with minimal rain over the year, hydrophobic self-cleaning coatings which eliminate the need to use water or chemicals may be preferred over hydrophilic coatings, as hydrophobic coatings are more proactive measures that prevent the accumulation of rain-carried dirt in the first place.
The self-cleaning action of hydrophobic coatings (commonly known as the “Lotus Effect”) arises from their high water contact angles. Water on these surfaces can form almost spherical droplets which do not adhere to the surface at all as they roll away very easily, carrying dust and dirt with them. Dirty water falling onto a hydrophobic coating may thus be rapidly removed before it can evaporate. Droplets tend to roll only on surfaces with very high contact angles (θc). If the contact angle is very high, e.g., θc>˜140 or 160°, the surface can be referred to as superhydrophobic.